1. Field of the Invention
This invention relates generally to pressure relief valves. The invention specifically relates to, but is not limited to, pilot operated pressure relief valves utilizing a pressure tap for sensing the pressure within the nozzle of the valve. This invention also specifically relates to pressure relief valves of all types which are subject to clogging.
2. Description of the Prior Art
Existing pilot operated pressure relief valve designs utilizing an inlet pressure tap which is an integral part of the valve body must do so with either a "semi-nozzle" inlet design or one where the valve body itself forms the entire flow passage. In the semi-nozzle inlet design the inlet flow passage or primary pressure zone is formed in the valve body by the body itself and a seat or seat ring. The semi-nozzle design is illustrated in FIG. 1 of the drawings. The seat ring usually forms only a short portion of the inlet length through the valve, the valve body comprises the major length. The design permits a pitot tube to be inserted through the valve body wall into the inlet flow stream. This allows the pilot control of the pilot operated pressure relief valve to sense inlet pressure directly in the valve inlet. This is a desirable location to sense inlet pressure in most applications as it permits this type of pilot operated pressure relief valve to be self-contained, i.e., no auxiliary pitot connection is required to be installed elsewhere in the system. Since this type of pilot operated pressure relief valve is complete in itself, it may be installed without further consideration to piping requirements.
Due to the fact that the semi-nozzle valve inlet has two components comprising the inlet flow stream, the body and seat ring, there is, by necessity, a mechanical connection between these two parts. This connection may be a direct connection by threading, welding, etc., or the seat ring may be held in place by other components designed so as to directly load it against the valve body. The former method often proves to be difficult or costly to remove because of corrosion on the threaded connection or because removal of the weld poses difficulties in completing such a task successfully. In addition to the difficulty in removing the seat ring, the semi-nozzle design requires that where a carbon steel valve body is used, the inlet passage be comprised of non-corrosion resistant material unless the entire valve body is made of a more expensive corrosion-resistant material.
Only pilot operated pressure relief valves of the semi-nozzle design could employ an integral pitot tube in the inlet flow passage. Although full nozzle inlet pilot operated pressure relief valve designs (illustrated in FIG. 2) have been widely used, this design could not adopt the integral pitot tube principle since no portion of the valve body communicated directly with the inlet flow passage. A separate collar for mounting the pitot tube to the underside of the nozzle is shown in U.S. Pat. No. 3,791,553.
In the full nozzle design, only the nozzle is subjected to inlet pressure and flow. Attempts have been made to insert a pitot tube in a full nozzle valve through that portion of the full nozzle that extends below the valve body. With this method a full nozzle valve may integrally employ a pitot tube but several difficulties arise from this method. Due to the fact that the full nozzle is entirely a separate component from the valve body, a mechanical connection between the two is required. Typically, a screw thread on the full nozzle exterior engages a screw thread on the valve body at relative positions. The nozzle is therefore attached to the valve body by this connection. The screw thread does not provide for any angular adjustment of the nozzle relative to the body; therefore, the nozzle does not have any predetermined radial position relative to the valve body.
Should a full nozzle employ a pitot tube in the portion that extends beyond the valve body, no predetermined position for the pitot tube connection can be assumed. The installed position may be such that connecting the pilot control is difficult or impossible should the connection be located adjacent to a stud bolt position. There may be as many as 20 stud bolts which leave very little room for the pitot tube connection to be positioned between. This position could be predetermined by a trial fitting of the nozzle to the body to allow exact placement of the pitot tube drilling and tapping in the desired position. However, this is cumbersome from a manufacturing standpoint and would not permit any subsequent machining on the body or full nozzle that would alter this unique physical relationship.
A problem which affects pressure relief valves of all types, both pilot operated and spring operated, such as that disclosed in my U.S. patent application Ser. No. 276,934 filed June 24, 1981, is the clogging of the nozzles in "dirty" environments. In numerous applications, the fluid within the pressure vessel contains contaminates such as solid media or the fluid itself is subject to solidification. In such applications a coating of solids may begin to form around the interior surface of the nozzle of the pressure relief valve. If the solid material builds up, the inner diameter of the nozzle may even become completely clogged or sealed by the solidified material. When this occurs, the operation of the pressure relief valve may become completely impaired with potentially dangerous consequences. One approach to solving the unclogging problem is to inject high pressure steam through the valve body and directed to the valve seat. While this may clear the seat itself, it will not unclog the interior of the nozzle. The only way to clear the interior surface of the nozzle would be a shut down of the pressure vessel and a disassembly of the valve. Obviously, this raises severe down time problems and is not a task lightly or frequently undertaken.